The present invention generally relates to fuel injector assemblies and, more particularly, to a method for assembling and hardening a ball plug in a counterbore of a fuel injector nozzle assembly.
As is known in the art, the nozzle assemblies of fuel injectors contain drilled passages that must be plugged after machining. FIG. 1 illustrates a cross-section of one such prior art counterbored passage 10 formed into the body 12 of a fuel injector nozzle 14. The body 12 is made, for example, from H13 steel and is hardened (for example, core hardened and gas nitrided by means of processes well-known in the art). The core hardening and gas nitriding process infiltrates the crystalline structure of the steel body 12 to an average depth, as can clearly be seen in the areas 16.
After the core hardening and gas nitriding process is performed, a ball plug 18 is press fit into the counterbored passage 10. The ball plug 18 may be made, for example, from annealed 52100 steel. The press-fitting operation employs enough force to squeeze the distal end of the ball plug 18 into the reduced diameter section of the passage 10 counterbore, causing tearing of the ball plug 18 material in the process. This results in the distal end of the ball plug 18 not forming a tight seal with the walls of the passage 10, as can be seen at the gaps 20.
During operation of the fuel injector nozzle 14, the passage 10 is filled with fuel, which reaches a relatively high pressure (25 ksi or 375 lbf, for this prior art injector) during an injection event. The existence of the gaps 20 significantly increases the surface area upon which this pressure acts, resulting in an unacceptable failure rate of ball plugs 18 being forced out of the passage 10.
There is therefore a need for a method for assembling and heat treating a ball plug in a counterbore of a fuel injector nozzle assembly which significantly increases the pressure required to force the ball plug 18 out of the passage 10. The present invention is directed toward meeting this need.
According to the method of the present invention, the ball plug is press fit into the counterbored passage of a fuel injector nozzle assembly prior to hardening of the fuel injector nozzle assembly. A core hardening and gas nitriding process is then applied to the assembly, causing the ball plug to expand and the passage diameter to shrink. This results in a greatly increased seal between the ball plug and the passage, and substantial elimination of the gaps experienced therebetween in prior art fuel injector nozzle assemblies.